TextShape default values for Colors

After https://github.com/decentraland/unity-renderer/pull/2809

The default values of the following values should be:
```
optional Color3 shadow_color = 22; // default=(1.0,1.0,1.0)
optional Color3 outline_color = 23; // default=(1.0,1.0,1.0)
optional Color3 text_color = 24; // default=(1.0,1.0,1.0)
```
but it's taking non-colour, and its behaviour is undefined.

If there is no colour set to the TextShape, we should set the default at least once, and to accomplish that, we need to change the behaviour of the component.